This invention relates to an improved apparatus for measuring the current provided by a digitally controlled power supply when it is operating in a constant voltage mode and/or measuring the voltage provided when it is operating in a constant current mode. When the power supply operates in a constant voltage mode, voltage regulation means maintain the output voltage at a value represented by a digital voltage selection signal in response to the difference between the voltage represented by that signal and the output voltage; and when the power supply operates in a constant current mode, current regulation means maintain the output current at a value represented by a digital current selection signal in response to the difference between the current represented by that signal and the actual output current. As a power supply cannot operate in both modes at the same time, the voltage regulation means disables the current regulation means when the output current is less than that represented by the digital current selection signal, and the current regulation means disables the voltage regulation means when the output voltage is less than that represented by the digital voltage selection signal. The voltage regulation means and the current regulation means each provide a given mode signal when they are in control of the power supply.
Measurement of the output current when the power supply is operating in a constant voltage mode may be achieved by an ammeter, which is expensive and cumbersome, or it may be achieved by changing the digital current selection signal so as to reduce the current it represents until the power supply changes to a constant current mode and noting the current represented by the digital current selection signal at that time. Similarly, measurement of the output voltage when the power supply is operating in a constant current mode may be achieved by a voltmeter, which is expensive and cumbersome, or it may be achieved by changing the digital voltage selection signal so as to reduce the voltage it represents until the power supply changes to a constant voltage mode and noting the voltage represented by the digital voltage selection signal at that time. In many applications, changing the mode of operation of the power supply as is required by these methods of measuring current and voltage is highly objectionable.